A description of your process might explain why you need continuous measurements of bulk density. However, it is important to bear in mind that bulk density will vary through the bin in both vertical and horizontal planes. If you have core / funnel flow this horizontal variation will be pronounced as a result of the non flowing material around the periphery of the bin having a higher bulk density than the dilated material being drawn down the flow channel in the middle.

I suspect that you may be trying to detect the effects of segregation (and hence fluctuations in bulk density) through the discharge of the vessel - but I am happy to be clarified on this point!

If you are experiencing a high degree of variability in bulk density for your coffee granules and fine granules I would recommend that your attention be turned toward the possibility of the material being degraded en route to the vessel that you are interested in.

You will not be able to obtain meaningful values for bulk density that could be used to feed back into a process control system.

Richard Farnish

Consulting Engineer

The Wolfson Centre for Bulk Solids Handling Technology

University of Greenwich

mailto: R.J.Farnish@gre.ac.uk

http://WWW.BULKSOLIDS.COM ■

The question suggests that bulk density variations are being experienced in the discharge from a hopper. Richard identifies possible causes by way of granule degradation through handling attrition and/or segregation. A further possibility is that flow pressure variations due to the collapse of dynamic arches within a non-mass flow channel are giving rise to dilation changes that show as density fluctuations in the emerging product. It would seem highly likely that the existing installation is of non-mass flow construction for these density variations to arise.

Taking the view that it is better to cure the problem than measure the effect, perhaps a better approach is to review the equipment design to see how segregation may be mitigated and a smoother flow stream achieved. A mass flow system produces a far more consistent density condition that prevails in a non-mass flow stream and it also reduces segregation effects. Fortunately, coffee granules, if kept dry, are relatively free flowing. This feature allows them to segregate easily during handling but also enables flow inserts to be used that change the pattern of fill and discharge. This is a relatively specialised area of flow technology in which we would be pleased to advise on receipt of details of the existing equipment and operation. Some of the background is given in my book: - ‘User Guide to Segregation’, published by the British Materials Handling Board and technical publications on the use of flow inserts are available from lyn@ajax.co.uk.

Lyn Bates ■